A well known issue in GPS positioning is that the determination of the vertical coordinate—the height—is two to three times less precise than the horizontal ones. Two main reasons explain this fact: the GPS satellite sky distribution, in which no satellite is visible under the horizon, and the receiver clock error, which is highly correlated with the height component (R. Santerre, “Impact of GPS satellite sky distribution,” Manuscripta Geodaetica, 16(1), 28-53, (1991)).
To achieve better precision of the vertical coordinate in relative GPS positioning, a novel GPS architecture in which all the antennas are connected to a single GPS receiver is proposed in Santerre et al. (R. Santerre, and G. Beutler. “A proposed GPS method with multi-antennae and single receiver,” Bulletin Géodésique, 67(4), 210-223, (1993)). With this configuration, because only one GPS receiver is used, receiver clock errors are eliminated by single differentiation between antennas. Simulations predict two to three times improvement in the precision of the vertical position determination. Reaching millimetric vertical precision is important for applications such as deformation monitoring of civil engineering structures, e.g. dams or bridges. In this context, two main issues must be resolved to successfully implement the multi-antenna-to-one-receiver system. Firstly, as opposed to conventional GPS survey in which each antenna is separated by only a few metres from its associated receiver, in this case the distance between the antennas and the single receiver can reach several kilometres. Secondly, height precision improvement can only be reached if the relative propagation delay between the antennas and the receiver is monitored at the millimetre level. Optical fiber links are important components to address these issues. Several manufacturers already offer GPS-over-fiber solutions but these do not include real time monitoring of propagation delays. Additionally, since high precision applications rely on carrier phase measurements, phase stability is important. Therefore, a proper choice of components and measurement of phase stability must be performed. However, the major drawback of the system disclosed relates to the calibration of the system. In the system envisioned by Santerre et al, calibration of relative signal delay throughout the hardware (antenna, cables, receiver) is performed once, before the final deployment, using the zero-baseline configuration, where only one antenna is used, and the use of suitable low-thermal dilatation optical fiber is recommended so that variations in the fiber links lengths due to temperature changes are minimized. Such specialty fibers are however costly and not always convenient, which may limit the practicality of this system.
Systems taking into account fiber length thermal variations are known in other fields, such as the high precision timing control of a radiotelescope, as exemplified by Cliche et al. (J. Cliche, and B. Shillue, “Precision timing control for radioastronomy: maintaining femtosecond synchronization in the Atacama Large Millimeter Array”, IEEE Control Systems Magazine, 26(1), pp. 19-26 (2006)). Cliche et al. propose a real time calibration system for an optical fiber link to be used in radioastronomy. The system uses two synchronized laser (a master and a slave) in order to perform interferometry measurements and to adjust an optical fiber length in real time. The goal is to have a femtosecond synchronisation system. However, even if such system achieve a very high accuracy over long distance (up to 18 km), the cost of building and maintaining such system is very high and not appropriate or viable for most high precision GPS applications.
There is therefore a need for an improved architecture which addresses at least some of the above-mentioned drawbacks of prior art systems.